A "Goldilocks Zone" for Recruiting BET Proteins with Bromodomain-1-Selective Ligands.

Synthetic genome readers/regulators (SynGRs) are bifunctional molecules that are rationally designed to bind specific genomic sequences and engage cellular machinery that regulates the expression of targeted genes. The prototypical SynGR1 targets GAA trinucleotide repeats and recruits the BET family of transcriptional regulatory proteins via a flexibly tethered ligand, JQ1. This pan-BET ligand binds both tandem bromodomains of BET proteins (BD1 and BD2). Second-generation SynGRs, which substituted JQ1 with bromodomain-selective ligands, unexpectedly revealed that BD1-selective ligands failed to functionally engage BET proteins in living cells despite displaying the ability to bind BD1 in vitro. Mechanistically, recruiting a BET protein via BD1- or BD2-selective SynGRs should have resulted in indistinguishable functional outcomes. Here we report the conversion of inactive BD1-targeting SynGRs into functional gene regulators by a structure-guided redesign of the chemical linker that bridges the DNA-binding molecule to the highly selective BD1 ligand GSK778. The results point to an optimal zone for positioning the BD1-selective ligand for functional engagement of BET proteins on chromatin, consistent with the preferred binding of BD1 domains to distal acetyllysine residues on histone tails. The results not only resolve the mechanistic conundrum but also provide insight into domain-selective targeting and nuanced design of chemo probes and therapeutics.